shmop(2)                         UNIX System V                         shmop(2)


NAME
      shmop: shmat, shmdt - shared memory operations

SYNOPSIS
      #include <sys/types.h>
      #include <sys/ipc.h>
      #include <sys/shm.h>

      void *shmat(int shmid, void *shmaddr, int shmflg);

      int shmdt (void *shmaddr);

DESCRIPTION
      shmat attaches the shared memory segment associated with the shared
      memory identifier specified by shmid to the data segment of the calling
      process.  The segment is attached at the address specified by one of the
      following criteria:

            If shmaddr is equal to (void *) 0, the segment is attached at the
            first available address as selected by the system.

            If shmaddr is not equal to (void *) 0 and (shmflg&SHMRND) is true,
            the segment is attached at the address given by (shmaddr - (shmaddr
            modulus SHMLBA)).

            If shmaddr is not equal to (void *) 0 and (shmflg&SHMRND) is
            false, the segment is attached at the address given by shmaddr.

      shmdt detaches from the calling process's data segment the shared memory
      segment located at the address specified by shmaddr.

      The segment is attached for reading if (shmflg&SHMRDONLY) is true
      {READ}, otherwise it is attached for reading and writing {READ/WRITE}.

      shmat fails and does not attach the shared memory segment if one or more
      of the following are true:

      EINVAL         shmid is not a valid shared memory identifier.

      EACCES         Operation permission is denied to the calling process [see
                     intro(2)].

      ENOMEM         The available data space is not large enough to
                     accommodate the shared memory segment.

      EINVAL         shmaddr is not equal to zero, and the value of (shmaddr -
                     (shmaddr modulus SHMLBA)).  is an illegal address.

      EINVAL         shmaddr is not equal to zero, (shmflg&SHMRND) is false,
                     and the value of shmaddr is an illegal address.




10/89                                                                    Page 1

shmop(2)                         UNIX System V                         shmop(2)


      EMFILE         The number of shared memory segments attached to the
                     calling process would exceed the system-imposed limit.

      EINVAL         shmdt fails and does not detach the shared memory segment
                     if shmaddr is not the data segment start address of a
                     shared memory segment.

SEE ALSO
      intro(2), exec(2), exit(2), fork(2), shmctl(2), shmget(2).

DIAGNOSTICS
      Upon successful completion, the return value is as follows:

            shmat returns the data segment start address of the attached shared
            memory segment.

            shmdt returns a value of 0.

      Otherwise, a value of -1 is returned and errno is set to indicate the
      error.

NOTES
      The user must explicitly remove shared memory segments after the last
      reference to them has been removed.






























Page 2                                                                    10/89